CN104380203A - Photon source, metrology apparatus, lithographic system and device manufacturing method - Google Patents

Abstract

A laser driven light source comprises laser (52) and focusing optics (54). These produce a beam of radiation focused on a plasma forming zone within a container (40) containing a gas (e.g., Xe). Collection optics (44) collects photons emitted by a plasma (42) maintained by the laser radiation to form a beam of output radiation (46). The plasma has an elongate form (L>d) and the collecting optics is configured to collect photons emerging in the longitudinal direction from the plasma. The brightness of the plasma is increased compared with sources which collect radiation emerging transversely from the plasma. A metrology apparatus using the light source can achieve greater accuracy and/or throughput as a result of the increased brightness. Back reflectors may be provided. Microwave radiation may be used instead of laser radiation to form the plasma.

Description

Photon source, measuring device, photolithography system and device manufacturing method

Cross References to Related Applications

This application claims the benefit of US Provisional Application 61/658,654, filed June 12, 2012, which is hereby incorporated by reference in its entirety.

technical field

The present invention relates to plasma-based photon sources. Such a source can be used, for example, in methods to provide high-intensity illumination and can be used, for example, in metrology that can be used in the manufacture of devices by lithography and methods of manufacturing devices using lithography.

Background technique

The photon source according to the invention can be applied in a wide range of situations. As an exemplary application, we will describe the invention as a light source in metrology. As a specific field of application of metrology, for the sake of example we will refer to metrology during the fabrication of devices by photolithography. The terms "light" and "source of light" may be used conveniently to refer to the radiation produced as well as the photon source itself, and are not meant to be limited to radiation having visible wavelengths.

A lithographic apparatus is a machine that applies a desired pattern to a substrate, usually a target portion of the substrate. For example, lithographic equipment may be used in the manufacture of integrated circuits (ICs). In this case, a patterning device, alternatively referred to as a mask or reticle, may be used to generate the circuit pattern to be formed on the individual layers of the IC. The pattern can be transferred onto a target portion (eg, comprising a portion, one or more dies) on a substrate (eg, a silicon wafer). Typically, the pattern is transferred by imaging the pattern onto a layer of radiation-sensitive material (resist) disposed on the substrate. Typically, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatuses include: so-called steppers, in which each target portion is irradiated by exposing the entire pattern onto the target portion at a time; and so-called scanners, in which In so-called scanners, each target portion is irradiated by scanning the pattern with a radiation beam in a given direction (the "scanning" direction) while synchronously scanning the substrate in a direction parallel or anti-parallel to this direction. The pattern can also be transferred from the patterning device to the substrate by imprinting the pattern onto the substrate.

During lithography, it is often desirable to take measurements of the resulting structures, eg for process control and verification. A variety of tools are known for making such measurements, including scanning electron microscopes, which are often used to measure critical dimension (CD), and specialized tools for measuring overlay (the alignment accuracy of two layers in a device). Recently, various forms of scatterometers have been developed for use in the field of lithography. These devices direct a beam of radiation onto a target and measure one or more properties of the scattered radiation. From these measured properties, properties of interest of the target can be determined.

In one commercially available measurement device, the light source is a xenon (Xe) arc discharge lamp. The light from this lamp is imaged onto the measurement target through the illumination branch of the device sensor, the last stage of which consists of an objective lens with a high numerical aperture. The measurement spot can for example have a diameter of 25 μm. The spectral distribution of radiation may be broadband or narrowband in nature, with wavelengths in the near-infrared, visible and/or ultraviolet bands. The time required for each measurement depends in practice on the brightness of the light source at a given wavelength or band. It is expected that future devices will be able to provide increased spectral bandwidth and provide sensor designs with low transmission, while keeping measurement times constant or shorter. Significant improvement in source brightness is required in order to achieve these requirements.

Increased brightness cannot be achieved simply by increasing the total source power. In order to increase brightness, higher power must be delivered into the same small spot size. Etendue is a measure of how "unfolded or spread out" light is in an optical system. It is a fundamental property of an optical system that the "Etendue" never decreases as it passes through the system. The optical etendue at the target side of the optical system in the metrology device is very small (due to the small spot size). Therefore, the light source must deliver all of its energy at very small etendue in order to provide a true increase in usable brightness.

Plasma-based photon sources, such as laser-driven light sources (LDLS), provide higher brightness. Plasma can be generated in a gaseous medium by applying energy using a discharge and by applying laser energy. However, plasmas have a limited physical range, and increasing brightness remains a challenge for these sources.

Contents of the invention

The present invention aims to provide a photon source with high brightness by alternative means.

The present invention provides a plasma-based photon source device in a first aspect, comprising: a container for containing a gas environment; a drive system for generating radiation, hereinafter referred to as drive radiation, and the drive system for forming the drive radiation into at least one beam focused on a plasma formation region within the vessel; and collection optics for collecting photons emitted by the plasma held at the plasma site by the radiation beam , and for forming the collected photons into at least one output radiation beam. The drive system is configured to maintain the plasma in an elongated shape having a length along a longitudinal axis substantially greater than the plasma in at least one direction transverse to the longitudinal axis and wherein the collection optics are configured to collect photons emitted from the plasma from one end of the plasma along the longitudinal axis.

The drive system may comprise at least one laser for generating the radiation beam, wherein the radiation beam has a wavelength eg in the infrared or visible light band. Therefore, the present invention is suitable for application to laser-driven light sources. Alternatively, the drive system may be arranged to generate radiation in the microwave range. In either case, the drive system may be considered to be a drive optics system, applying eg infrared optics or microwave optics as the case may be.

As mentioned, the new photon source can be applied in metrology, for example in photolithography. In another aspect the invention provides a method of measuring properties of a structure formed on a substrate by a photolithographic process, said method comprising the steps of using a photon source according to the first aspect of the invention as described above The output radiation illuminates the structure; the radiation diffracted by the structure is detected; and one or more properties of the structure are determined from the properties of the diffracted radiation.

The invention also provides an inspection apparatus for measuring properties of structures on a substrate, said apparatus comprising: a support for having said structures thereon; an optical system for illuminating said structures under predetermined illumination conditions. said structure, and for detecting a predetermined portion of radiation which is diffracted by the constituent target structure under said illumination conditions; and a processor arranged to process information characterizing the detected radiation to obtain the structure's measurement of properties. The optical system comprises a photon source device according to the invention described above.

The present invention also provides a lithography system comprising a lithography apparatus comprising: an illumination optics system arranged to illuminate a pattern; a projection optics system arranged to illuminate the pattern An image of is projected onto a substrate; and an inspection apparatus according to an embodiment of the present invention as described above. The lithographic apparatus is arranged to use measurements from the inspection apparatus when applying the pattern to other substrates.

The present invention also provides a method of manufacturing a device in which a device pattern is applied to a series of substrates using a photolithographic process, the method comprising: inspecting at least one composite target structure using an inspection method according to claim 12, wherein the The at least one composite target structure is formed as part of a device pattern on at least one of the substrates or is formed next to the device pattern on at least one of the substrates; and according to the inspection method The results control the lithography process for subsequent substrates.

Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention are described in detail hereinafter with reference to the accompanying drawings. It should be noted that the invention is not limited to the specific examples described herein. Such embodiments are given herein for the purpose of illustration only. Additional embodiments will be apparent to those skilled in the relevant art from the teachings contained herein.

Description of drawings

The accompanying drawings, which are incorporated in and form a part of this specification, illustrate the invention and, together with the description, further serve to explain the principles of the invention and enable those skilled in the relevant art to make and use the invention.

Figure 1 shows a lithographic apparatus according to an embodiment of the invention.

Figure 2 illustrates a lithographic cell or cluster according to an embodiment of the invention.

Figure 3 includes an exemplary view of an optical device containing a photon source, in this example in the form of a scatterometer for use in metrology.

Fig. 4 is a schematic view of a new photon source used in the device shown in Fig. 3 according to a first embodiment of the present invention.

Figure 5 is a graph showing experimental data regarding the relative brightness of elongated plasmas when viewed longitudinally and laterally.

Fig. 6 is a schematic view of a new photon source used in the device shown in Fig. 3 according to a second embodiment of the present invention.

7(a) and (b) are exemplary views of a new photon source used in the apparatus shown in FIG. 3 according to a third embodiment of the present invention.

The features and advantages of the present invention will be more readily understood from the detailed description set forth below when taken in conjunction with the accompanying drawings, in which like reference letters designate corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number.

detailed description

This specification discloses one or more embodiments that incorporate the features of this invention. The disclosed embodiments are merely illustrative of the invention. The scope of the invention is not limited to the disclosed embodiments. The invention is defined by the appended claims.

The embodiments and references in this specification to "one embodiment," "an embodiment," "example embodiment" and the like mean that the embodiments may include a particular feature, structure, or characteristic, but each embodiment It may not be necessary to include that particular feature, structure or characteristic. Additionally, these terms are not necessarily referring to the same embodiment. Furthermore, when a particular feature, structure, or characteristic is described in conjunction with an embodiment, it should be understood that it is within the knowledge of those skilled in the art, whether explicitly described or not, to implement such feature, structure, or characteristic in conjunction with other embodiments. structure or property.

Embodiments of the invention may be implemented as hardware, firmware, software or any combination thereof. Embodiments of the invention can also be implemented as instructions stored on a machine-readable medium, which can be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (eg, a computing device). For example, a machine-readable medium may include read-only memory (ROM); random-access memory (RAM); magnetic disk storage media; optical storage media; flash memory devices; signal, digital signal, etc.) and others. Also, firmware, software, routines, instructions may be described herein as performing certain actions. It should be understood, however, that this description is for convenience only and that such actions are actually caused by a computing device, processor, controller, or other means for executing firmware, software, routines, instructions, or the like.

Before describing such embodiments in more detail, however, it is instructive to illustrate an example environment in which embodiments of the invention may be implemented.

Figure 1 schematically shows a lithographic apparatus LA. The apparatus comprises: an illumination system (illuminator) IL configured to condition a radiation beam B (e.g. UV radiation or DUV radiation); a patterning device support or support structure (e.g. a mask table) MT configured for For supporting the patterning device (such as a mask) MA, and connected to the first positioning device PM configured to accurately position the patterning device according to specific parameters; the substrate table (such as a wafer table) WT, which is configured to hold a substrate (e.g. a resist-coated wafer) W connected to a second positioner PW configured to precisely position the substrate according to specific parameters; and a projection system (e.g. a refractive projection lens system) PS, It is configured for projecting the pattern imparted to the radiation beam B by the patterning device MA onto a target portion C of the substrate W (eg comprising one or more dies).

The illumination system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic, or other types of optical components, or any combination thereof, to direct, shape, or control the radiation.

The patterning device support holds the patterning device in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions such as, for example, whether the patterning device is held in a vacuum environment. The patterning device support may employ mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The patterning device support may be a frame or a table, for example, which may be fixed or movable as desired. The patterning device support may secure the patterning device in a desired position (eg relative to the projection system). Any use of the terms "reticle" or "mask" herein may be considered synonymous with the more general term "patterning device."

The term "patterning device" as used herein should be broadly construed to mean any device that can be used to impart a radiation beam with a pattern in its cross-section so as to form a pattern in a target portion of a substrate. It should be noted that the pattern imparted to the radiation beam may not correspond exactly to the desired pattern on the target portion of the substrate (eg if the pattern includes phase shifting features or so called assist features). Typically, the pattern imparted to the radiation beam will correspond to a specific functional layer in a device formed on the target portion, such as an integrated circuit.

The patterning device can be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography and include mask types such as binary, alternating phase-shift, attenuated phase-shift, and various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be independently tilted to reflect an incident radiation beam in different directions. The tilted mirrors impart a pattern to the radiation beam reflected by the mirror matrix.

The term "projection system" as used herein may be broadly interpreted to include any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic, and electrostatic optical systems, or any combination thereof, as for all as appropriate for the exposure radiation used, or for other factors such as the use of immersion liquid or the use of a vacuum. Any use of the term "projection lens" herein may be considered synonymous with the more general term "projection system".

As shown here, the device is transmissive (eg, employs a transmissive mask). Alternatively, the device may be reflective (eg, employing a programmable mirror array of the type described above, or employing a reflective mask).

The lithographic apparatus may be of the type with two (dual-stage) or more substrate stages (and/or two or more mask stages). In such "multi-stage" machines, additional tables may be used in parallel, or one or more other tables may be used for exposure while preparatory steps are being performed on one or more tables.

The lithographic apparatus may also be of the type in which at least a part of the substrate may be covered by a liquid having a relatively high refractive index, such as water, in order to fill the space between the projection system and the substrate. The immersion liquid can also be applied to other spaces in the lithographic apparatus, such as the space between the mask and the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems. The term "immersion" as used here does not mean that the structure (such as the substrate) must be

Immersed in a liquid, but only means that the liquid is between the projection system and the substrate during exposure.

Referring to FIG. 1 , an illuminator IL receives a radiation beam from a radiation source SO. The source and lithographic apparatus may be separate entities (eg when the source is an excimer laser). In this case, the source is not considered to form part of the lithographic apparatus, and the radiation beam is diverted from the The source SO is passed to the illuminator IL. In other cases, the source may be an integral part of the lithographic apparatus (eg when the source is a mercury lamp). The source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AD for adjusting the angular intensity distribution of the radiation beam. Typically, at least the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in the pupil plane of the illuminator can be adjusted. In addition, the illuminator IL may include various other components, such as an integrator IN and a concentrator CO. The illuminator can be used to condition the radiation beam to have a desired uniformity and intensity distribution in its cross-section.

The radiation beam B is incident on the patterning device (eg mask) MA held on a patterning device support (eg mask table MT) and is patterned by the patterning device. After having passed through the patterning device (eg mask) MA, the radiation beam B passes through a projection system PS which focuses the radiation beam onto a target portion C of the substrate W. With the help of a second positioner PW and a position sensor IF (e.g. an interferometric device, a linear encoder, a two-dimensional encoder or a capacitive sensor), the substrate table WT can be moved precisely, for example in order to place different target parts C is positioned in the path of said radiation beam B. Similarly, the first positioner PM and a further position sensor (not explicitly shown in FIG. The path of beam B precisely positions patterning device (eg, mask) MA. Typically movement of the patterning device support (eg mask table) MT can be achieved with the aid of a long stroke module (coarse positioning) and a short stroke module (fine positioning) forming part of said first positioner PM. Similarly, movement of the substrate table WT may be achieved using a long-stroke module and a short-stroke module forming part of the second positioner PW. In the case of a stepper (as opposed to a scanner), the patterning device support (eg mask table) MT may only be associated with a short-stroke actuator, or may be fixed.

Patterning device (eg mask) MA and substrate W may be aligned using mask alignment marks M1 , M2 and substrate alignment marks P1 , P2 . Although the substrate alignment marks are shown occupying dedicated target portions, they may be located in spaces between target portions (these are known as scribe line alignment marks). Similarly, where more than one die is disposed on the patterning device (eg mask) MA, the mask alignment marks may be located between the dies. Small alignment marks may also be included within the die, between device features, in which case it is desirable that the marks be as small as possible and not require any different imaging or processing conditions than adjacent features. An alignment system that detects alignment marks will be described further below.

The device shown can be used in at least one of the following modes:

1. In step mode, the entire pattern imparted to the radiation beam is projected onto a target portion C at once while holding the patterning device support (e.g. mask table) MT and substrate table WT substantially stationary (ie, a single static exposure). The substrate table WT is then moved in the X and/or Y direction so that different target portions C can be exposed. In step mode, the maximum size of the exposure field limits the size of the target portion C imaged in a single static exposure.

2. In scanning mode, the pattern imparted to the radiation beam is projected onto a target portion C while simultaneously scanning the patterning device support (e.g. mask table) MT and substrate table WT (i.e. single dynamic exposure). The velocity and direction of the substrate table WT relative to the patterning device support (eg mask table) MT can be determined by the (de-)magnification and image inversion characteristics of the projection system PS. In scanning mode, the maximum size of the exposure field limits the width of the target portion in a single dynamic exposure (in the non-scanning direction), while the length of the scanning movement determines the height of the target portion (in the scanning direction). ).

3. In another mode, the patterning device support (e.g. mask table) MT holding the programmable patterning device is held substantially stationary and the pattern imparted to the radiation beam is projected onto the target portion C At the same time, the substrate table WT is moved or scanned. In this mode, a pulsed radiation source is typically employed and the programmable patterning device is updated as required after each movement of the substrate table WT, or between successive radiation pulses during scanning. This mode of operation is readily applicable in maskless lithography using programmable patterning devices, such as programmable mirror arrays of the type described above.

Combinations and/or variations of the above described modes of use, or entirely different modes of use may also be employed.

The lithographic apparatus LA is of the so-called dual-stage type, which has two substrate tables WTa, WTb and two stations - an exposure station and a measurement station, between which the substrate tables can be exchanged. While one substrate on one substrate table is being exposed at the exposure station, another substrate can be loaded onto another substrate table at the measurement station and various preparatory steps performed. Said preliminary steps may include planning the surface control of the substrate using the level sensor LS and measuring the position of the alignment marks on the substrate using the alignment sensor AS. This can substantially increase the productivity of the plant. If the position sensor IF cannot measure the position of the substrate table when the substrate table is at the measurement station as well as at the exposure station, a second position sensor may be provided to enable the position of the substrate table to be tracked at both stations.

As shown in Figure 2, the lithographic apparatus LA forms part of a lithographic cell LC (sometimes referred to as a lithocell or a lithocluster), which also includes components for performing pre- and post-exposure processing on a substrate. device of. Typically these include spin coaters SC to deposit the resist layer, developers DE to develop the exposed resist, chill plates CH and bake plates BK. A substrate handler or robot RO picks up substrates from the input/output ports I/O1, I/O2 and moves them between different processing tools before transferring them to the feed table LB of the lithographic tool. These devices, often collectively referred to as a track, are under the control of a track control unit TCU which itself is controlled by a supervisory control system SCS which also controls the lithographic apparatus via the lithographic control unit LACU. Accordingly, different devices can be operated to maximize productivity and process efficiency.

FIG. 3 is a schematic view of an optical device in the form of a scatterometer suitable for performing measurements in conjunction with the lithography cell of FIG. 2 . The apparatus can be used to measure critical dimensions of features formed by photolithography, can be used to measure overlap between layers, and the like. Product features or dedicated metrology targets are formed on the substrate W. The apparatus may be a stand-alone device or may be incorporated in a lithographic apparatus (for example at a measurement station) or a lithographic cell LC. The optical axis is indicated by a dotted line O, which has a number of branches running through the device. In this device, light emitted by a source 11 is directed onto a substrate W via a beam splitter 15 by means of an optical system comprising lenses 12 , 14 and an objective 16 . The lenses are arranged in a double sequence of 4F arrangement. Different lens arrangements may be used as long as such a lens arrangement is still capable of providing an image of the source onto the substrate and at the same time allows access to the intermediate pupil plane for space-frequency filtering. Thus, the range of angles over which radiation is incident on the substrate can be selected by defining a spatial intensity distribution in a plane representing the spatial spectrum of the substrate plane (herein referred to as the (conjugate) pupil plane). In particular, this can be done by inserting an aperture plate 13 of suitable form between the lenses 12 and 14 in the plane of the post-projected image which is the pupil plane of the objective. For example, as shown, aperture plate 13 may have different forms, two of which are labeled 13N and 13S, allowing selection of different modes of illumination. The illumination system in this example forms an off-axis illumination pattern. In the first illumination mode, the aperture plate 13N provides off-axis from a direction labeled "N (North)" (for illustration only). In a second illumination mode, aperture plate 13S is used to provide similar illumination, but from the opposite direction labeled "S (South)". Other illumination patterns can also be achieved by using different apertures. The remainder of the pupil plane is desirably dark, since any unnecessary light outside the desired illumination pattern will interfere with the desired measurement signal.

At least the 0 order and one of the +1 order and −1 order diffracted by the target on the substrate W are collected by the objective lens 16 and guided back through the beam splitter 15 . The second beam splitter 17 splits the diffracted beam into two measurement branches. In the first measurement branch, the optical system 18 uses the zeroth and first order diffracted beams to form a diffracted spectrum (pupil plane image) of the target on a first sensor 19 (eg CCD or CMOS sensor). Each diffraction order hits a different point on the sensor so that image processing can compare and contrast the diffraction orders. The pupil plane image captured by the sensor 19 may be used for a convergence metrology device and/or to normalize intensity measurements of the first order beam. The pupil plane image can also be used for many measurement purposes, such as reconstruction.

In the second measurement branch, the optical system 20, 22 forms an image of the object on the substrate W on a sensor 23, eg a CCD or CMOS sensor. In the second measurement branch, the aperture stop 21 is arranged in a plane conjugate to the pupil plane. The function of the aperture stop 21 is to block the zeroth order diffracted beams so that the image of the object formed on the sensor 23 is formed only by the -1 or +1 first order beams. Therefore, the image detected by the sensor 23 is called a "dark field" image. Note that the term "image" is used here in a broad sense. If only one of the -1 and +1 diffraction orders is present, the image of the grating lines will also not be formed.

The images captured by the sensors 19 and 23 are output to an image processor and controller PU whose functionality will depend on the particular type of measurement being made. Further details of the device and its applications can be found in the earlier patent applications mentioned in the introduction above. The present disclosure relates to the construction and operation of light source 11 to provide higher brightness than xenon arc lamps used in known devices.

Examples of scatterometers and techniques can be found in patent applications US2006/066855A1, WO2009/078708, WO2009/106279 and US2011/0027704A, which are hereby incorporated by reference in their entirety. Published patent application US 2011/204265A1, which is incorporated by reference in its entirety, discloses plasma-based light sources, including laser-driven light sources. It can be explained that the plasma can take an elongated shape, which increases the radiation area and increases brightness. Measures are described for reducing the longitudinal extent of the plasma for the purpose of increasing the brightness.

Figure 4 schematically shows the main components of a laser-driven photon source device. The central component is a container 40, such as a glass capsule, containing a predetermined gaseous environment or atmosphere. A suitable gas may be, for example, xenon (Xe) or a xenon-argon gas mixture. In this gaseous environment, a plasma 42 is created in the manner to be described, and the plasma emits light (more generally, radiation photons having a desired wavelength). Collection optics 44 form a radiation beam 46 which is coupled to an optical fiber 48 . Optical fiber 48 delivers the radiation to its desired point. The end of the optical fiber 48 forms the source 11 seen in FIG. 3 when the photon source is used as the source in the device of FIG. 3 . Collection optics 44 are shown here as simple lenses, but could of course be more complex in practical embodiments. Reflective optics may be used instead of refractive optics.

The plasma 42 in this embodiment is generated by applying driving radiation 50 , which is generated by a laser 52 in this example. Drive optics 54 focus the laser light into a converging beam 56 that reaches its narrowest point at the location where plasma 42 is desired to be formed and maintained. Laser 52 may be one of many different types of high power lasers available today or in the future. Examples include Nd:YAG lasers, CO ₂ lasers, diode lasers, fiber lasers. The drive optics 54 are shown here as simple lenses, but could of course be more complex in practical embodiments. Reflective optics may be used instead of refractive optics. Additional components can be provided in order to adjust the profile or spectral properties of the laser radiation. For example a beam expander may be used.

For example laser radiation may be in the infrared wavelengths, such as 700 to 2000 nm. Typically, the plasma will generate radiation at shorter wavelengths, for example down to 200 nm or less, in the infrared, visible and/or ultraviolet bands. In this plasma radiation is the desired wavelength for use in metrology equipment or other applications. A filter assembly 58 may be disposed in the optical path, for example to reduce the amount of infrared radiation entering collection optics 44 and/or filter 48 . Such filters may be placed inside container 40 and/or outside container 40 . They can also be integrated with the container wall and/or with other components of the collection optics 44 .

Although the laser energy 50 is very narrowly focused, the laser energy 50 is not necessarily sufficient to ignite the plasma from a cold start, the electrodes 60 and 62 are provided with appropriate power and control circuitry (not shown) to ignite the plasma. These electrodes may be similar to those used in conventional gas discharge lamps, but are only used during the start-up phase of operation.

In the view, the X, Y and Z axes are defined for descriptive reasons. The Z axis is aligned with the optical axis O. The Y direction is aligned with the electrodes 60,62. The X-axis runs across the electrodes and is normal to the plane of view. The apparatus may be constructed or mounted with these axes in any orientation convenient for its application. Note that there are no components blocking the optical path from the plasma 42 to the collection optics in the Z direction. In this example there is also nothing blocking the path of the light in the X direction (not shown in this view).

It will be noted that the plasma 42 , or at least the portion of the region of the plasma that produces the desired radiation, takes an elongated shape, having a generally cylindrical shape or the shape of a cigar. For the sake of explanation, we will refer to the shape of a cylinder. The length of the cylinder is L and its diameter is d. The actual plasma will consist of a cloud of elongated shape, centered over this cylindrical region. The collection optics 44 are arranged such that its optical axis O is aligned with the longitudinal direction of the plasma, in this example the Z direction. Therefore, the area of the plasma is shown as πd ² /4, which is the area of one end of the cylinder. When L is substantially greater than d, the depth through which photons can enter the collection optics through this small area is greater than when looking at the plasma laterally. This enables a higher brightness to be seen over that area for a plasma of a given size and intensity. In general, the etendue of a light source (or receiver) is the product of the area of the source (collector) and its exit (incidence) angle. As with any imaging system, the etendue of collection optics 44 is the product of the spot size multiple and its numerical aperture ^squared (NA2). The numerical aperture NA is determined by the incident angle Θ. The etendue of the radiating plasma will generally be greater than the etendue of the collection optics 44 . Collection optics 44 may be converged at an imaginary source point 64 halfway along the cylinder, as shown. In a practical example, the length L of the light-emitting plasma region 42 may be on the order of millimeters, such as 0.5 to 5 mm. The diameter d may be very small, eg in the range of 0.01 to 2 mm, for example in the range of 0.1 to 1 mm.

In fact, the plasma absorbs very little of the required radiation so that photons emitted from anywhere along the length L of the cylinder can travel in the incidence cone of the collection optics 44 to enter the optical fiber 48 . Thus, the plasma appears brighter (more luminous flux per unit area, per unit solid angle) than when viewed in the lateral direction compared to the lateral direction. While the known laser-driven light source described in US2011/204265A1 (herein incorporated by reference in its entirety) seeks to capture light emitted in the transverse direction, the new photon source captures light emitted in the longitudinal direction to exploit increased brightness and to a lesser extent plasma. While in known sources design measures were taken in an attempt to reduce the length L of the plasma to concentrate its power over a smaller length, in the new source the constraints on the shape of the plasma are relatively relaxed . While in some examples in earlier patent applications the plasma extends between the ignition electrodes in the longitudinal direction (hereafter we describe it as the Y direction), in the new source the plasma in normal operation is arranged such that the longitudinal direction The light rays are not obscured and can be captured by collection optics 44 . Similarly, while in other examples in the prior patent application the plasma extends along what we describe as the Z direction, this is blocked by the drive laser optics and useful light travels from the plasma along the X and Y directions. After the direction is emitted, it is captured by the curved mirror. Thus, all examples in the prior patents rely on trapping photons emitted laterally from the plasma.

Figure 5 shows experimental results confirming the enhanced brightness of the elongated plasma 42 across the entire wavelength spectrum when viewed in the longitudinal direction, but not in the transverse direction. The horizontal axis represents wavelength (λ), extending from 200 nm in the ultraviolet end to 850 nm in the near infrared. The vertical axis represents luminance (B) at each wavelength in units of 10 ⁴ W/m ² /nm/sr. The upper curve 70 shows the measured brightness when looking at the plasma longitudinally, while the lower curves 72, 74 (the two lines are very close together, shown overlapping) show the measured brightness when looking at the plasma laterally. Clearly, the portrait view exhibits greater brightness across the entire spectrum. In the visible and ultraviolet parts of the spectrum, the brightness increases by a factor of 5 or more. The plasma in this experiment had a diameter d of approximately 0.3 mm and a length L of approximately 1.5 mm.

Numerical modeling also confirms that brightness seen along the longitudinal direction will increase as a function of plasma length (relative to the transverse direction). For the first modeled example (with a plasma of diameter d = 300 μm (0.3 mm) and a half angle (Θ/2) of 50 mrad for the collection optics), the brightness increases very rapidly as a function of length L, at approximately Up to 10 times at 5mm length and up to about 15 times the upper limit. For the second modeling example with d = 1 mm and Θ/2 = 15 mrad, the increase in brightness rises more steadily, reaching a factor of 10 at about 10 mm.

It should be noted that the intensity profile of the radiation emitted by the plasma source may not be very uniform throughout the field of view of the collection optics 44 . Although the constraint on the plasma size is relaxed as described above, the entrance NA of the collection optics 44 should still be filled with radiation suitably uniformly. The larger the aspect ratio L/d of the plasma, the smaller the etendue within which the radiation will be uniformly distributed. For example when a photon source device is used to deliver a uniform light field through the aperture 13 in the device shown in Figure 13, it may be desirable to mix the light so that it is more uniform. Sufficient mixing will occur naturally within the fiber 48, or additional measures may be employed. Furthermore, the optical properties of the walls of the vessel 40 should be good enough in critical locations that they do not degrade the quality of the beam of light or the driving laser beam emitted from the plasma 42 to the collection optics 44 . The optical properties of the container walls should of course be considered during the design and construction of the collection optics 44 and focusing optics 54 . If desired, the functional elements of collection optics 44 and focusing optics 54 may be placed in container 40 and/or may be integrated with the walls of the container.

Figures 6 and 7 show further embodiments based on the principles just described. The same reference numerals as in Fig. 4 will be used for components having the same functions. The optical fibers 48 are omitted in Figures 6 and 7 only to create space. Unless otherwise stated, the options and properties described with respect to the example of FIG. 4 apply equally to the examples of FIGS. 6 and 7 .

The arrangement in Figure 6 is very similar to that of Figure 4, except that a retroreflective mirror 80 is arranged to reflect light 82 emitted in the reverse longitudinal direction back into the plasma. The reflector 80 may, for example, be spherical, curved about the focal point 64 with a radius r. Since the reverse longitudinal direction is also the direction of the incident laser beam 50, the reflector 80 is formed with holes of appropriate size to allow the laser beam to pass through. The reflector 80 may be provided inside or outside the container 40, and may be formed on the wall of the container 40 itself. Additional reflectors can be provided to capture laterally emitted photons and return them to the plasma. Examples of these will be explained in the example of FIG. 7 . Photons reflected by these side reflectors will not enter the collection optics with small etendue as described. However, they can assist the laser energy 50 in maintaining the plasma, and they can improve the uniformity of the plasma and thus the uniformity of the emitted radiation. As previously mentioned, the optical fiber 58 or equivalent may be arranged to tailor the spectral properties of the outgoing radiation beam 46 . In particular, optical fiber 58 may be configured to reduce or eliminate radiation 50 entering collection optics 44 from a laser (eg, infrared radiation).

Figure 7 shows two views of another modified embodiment. The plasma 42 and collection optics 44 are shaped and oriented as in the previous example, with a retroreflector 80 also provided. In this example, however, the plasma-driving laser radiation 50 is delivered transversely to the elongated plasma, for example in the X-direction. Additional "side" reflectors 84 may be provided to capture laterally emitted photons 86 and return them to the plasma. Opposite side reflectors 88 may be provided having apertures 89 allowing passage of laser radiation delivered by focusing optics 54 to form beam 90 . Photons reflected by these side reflectors will not enter the collection optics with small etendue as described. However, they can assist the laser energy 50 in maintaining the plasma, and they can improve the uniformity of the plasma and thus the uniformity of the emitted radiation. Reflectors 84 and 88 may be spherical, cylindrical, or may be a composite shape of both, depending on whether some redistribution (mixing) of energy in the longitudinal direction is desired. Reflector 84 may be provided with holes similar to those in reflector 88 to allow laser radiation to exit. Side reflectors can be positioned close to the electrode sites if desired. As previously mentioned, the reflector can be inside the container 40 (as shown) or outside the container 40 . They can be integrated with the container wall.

Instead of being focused into an elongated beam aligned with the longitudinal direction, the laser radiation 50 is in this example spread out and focused into a beam 90 having a line-shaped focus or line focus, aligned with the desired plasma 42 size match. To this end, the focusing optics 54 essentially comprise cylindrical lenses, as shown, in order to form a converging beam 90 as seen in the Z direction in FIG. 7( b ). A beam expander 92 may be disposed between the laser 52 and the optics 54 to expand the beam to a desired width. Looking in the Y direction, as shown in Figure 7(a), beam 90 may be convergent as shown. Alternatively, beam 90 may be divergent when viewed in the Y direction and convergent when viewed in the Z direction. Multiple laser beams and lenses may be configured to generate a desired focus along the line of plasma 42 . If desired, driving radiation may be delivered in both the transverse and longitudinal directions, combining the features of FIGS. 4-6 and 7 .

In this second example, although the focusing optics may be more complex, there may be some advantages over the examples of FIGS. 4 and 6 . In principle, it will be seen that the longitudinal direction of the plasma is also the exit direction of useful radiation and is orthogonal to the axis of the ignition electrode, which in turn is orthogonal to the entry axis of the driving laser beam. Thus, such an arrangement has greater freedom from constraints in terms of mechanical and optical interference between these subsystems. For example, one advantage is that retroreflector 80 (if provided) does not require holes. Optionally, the second collection optics and second output can be taken from the "back" end of the plasma. The requirement for filtering to prevent laser radiation from entering collection optics 44 is also eliminated or reduced. Other changes and modifications discussed in the context of the previous example can also be applied to the example shown in FIG. 7 .

It is also noted that the principle of increasing brightness using radiation emitted in the longitudinal direction of the elongated plasma is not limited to the above-mentioned example of a laser-driven light source. Elongated plasmas can be produced by focusing other types of radiation, especially microwave radiation. By replacing laser 50 with a suitable microwave source or sources and focusing optics 54 with suitable microwave focusing optics, the advantages of forming elongated plasma 42 and using longitudinal emission can be applied. The principle of using longitudinal emission can be applied using a plasma formed by other means or methods, eg by an electric field.

Further embodiments according to the present invention are provided in the following numbered schemes:

1. A plasma-based photon source device comprising:

(a) Containers used to contain gaseous environments;

(b) a drive system for generating radiation (hereinafter referred to as drive radiation) and for forming said drive radiation into at least one beam focused on a plasma formation region within said vessel; and

(c) collection optics for collecting photons emitted by the plasma held at the plasma site by said radiation beam and for forming the collected photons into at least one output radiation beam,

wherein the drive optics are configured to maintain the plasma in an elongated shape having a length along a longitudinal axis substantially or significantly greater than at least one The diameter of the plasma in the direction, and wherein the collection optics are configured to collect photons exiting the plasma from one end of the plasma along the longitudinal axis.

2. Apparatus according to claim 1, wherein said drive system comprises at least one laser for generating said radiation beam.

3. A device according to claim 1 or 2, wherein the driving radiation has a wavelength predominantly in a first range, such as infrared wavelengths, and the output radiation has a wavelength predominantly in a second range different from the first range within wavelengths, such as visible light and/or ultraviolet radiation.

4. Apparatus according to any one of clauses 1-3, wherein the drive system is arranged for along the longitudinal axis at an end of the plasma opposite that from which the collected photons exit the plasma passing the driving radiation at one end of the .

5. Apparatus according to any one of clauses 1-4, wherein the drive system is arranged to deliver the drive radiation to the plasma formation site in a direction transverse to the longitudinal direction.

6. Apparatus according to clause 5, wherein the drive system is arranged to focus the drive radiation into a substantially line focus corresponding to the elongated shape of the plasma.

7. Apparatus according to any one of the preceding aspects, further comprising two or more electrodes positioned on opposite sides of the plasma formation site for igniting the plasma prior to operation, the The electrodes are positioned away from the longitudinal axis.

8. The apparatus of claim 7, wherein the electrodes are positioned on an axis orthogonal to the longitudinal axis.

9. The device according to clause 8, wherein the electrodes, the drive system and the collection optics are arranged on three mutually orthogonal axes.

10. Apparatus according to any one of the preceding aspects, further comprising a reflector positioned and shaped to reflect photons exiting longitudinally from opposite ends of the plasma back into the plasma.

11. Apparatus according to any one of the preceding aspects, further comprising one or more reflectors positioned and shaped to move along one or more Photons emerging from more directions are reflected back into the plasma.

12. A method of measuring properties of a structure that has been formed on a substrate by a photolithographic process, said method comprising the steps of:

(a) irradiating the structure with the output radiation of the photon source according to any one of schemes 1-11;

(b) detecting radiation diffracted by said structure; and

(c) determining one or more properties of the structure from properties of the diffracted radiation.

13. An inspection apparatus for measuring properties of structures on a substrate, the apparatus comprising:

(a) a support for a substrate having said structure thereon;

(b) an optical system for illuminating said structure under predetermined illumination conditions and for detecting a predetermined portion of radiation diffracted by the constituent target structure under said illumination conditions;

(c) a processor arranged to process information characterizing the detected radiation to obtain a measure of a property of the structure;

(d) wherein the optical system comprises the photon source device according to any one of schemes 1-11.

14. A photolithography system comprising:

(a) lithographic equipment, said lithographic equipment comprising:

(b) an illumination optics system arranged to illuminate the pattern;

(c) a projection optics system arranged to project an image of said pattern onto a substrate; and

(d) an inspection device according to scheme 13,

Wherein the lithographic apparatus is arranged to use measurements from the inspection apparatus when applying the pattern to other substrates.

15. A method of fabricating a device, wherein a photolithographic process is used to apply a device pattern to a series of substrates, the method comprising: inspecting at least one composite target structure using the inspection method according to claim 12, wherein the at least one forming a target structure formed as part of a device pattern on at least one of the substrates or beside a device pattern on at least one of the substrates; and controlling according to a result of the inspection method Photolithography process for subsequent substrates.

16. A plasma-based photon source apparatus comprising:

a container configured to contain a gaseous environment;

a drive system configured for generating drive radiation and for forming the drive radiation into at least one beam focused on a plasma formation region within the vessel, and

collection optics configured for collecting photons emitted by the plasma held at the plasma site by the radiation beam and for forming the collected photons into at least one output radiation beam,

wherein the drive optics are configured to maintain the plasma in an elongated shape having a length along a longitudinal axis substantially greater than in at least one direction transverse to the longitudinal axis the diameter of the plasma, and

Wherein the collection optics are configured to collect photons emerging from the plasma along a longitudinal axis from one end of the plasma.

17. The apparatus according to clause 16, wherein said drive system comprises at least one laser for generating said radiation beam.

18. The device of claim 16, wherein the drive radiation has a wavelength primarily in a first range of infrared wavelengths, and the output radiation has visible and/or ultraviolet radiation primarily in a different range than the first range wavelengths in the second range of .

19. The apparatus of claim 16, wherein the drive system is arranged to deliver the drive radiation.

20. The apparatus of clause 16, wherein the drive system is arranged to deliver the drive radiation to the plasma formation site in a direction transverse to the longitudinal direction.

21. The apparatus of clause 20, wherein the drive system is arranged to focus the drive radiation into a substantially line focus corresponding to the elongate shape of the plasma.

22. The device of embodiment 16, further comprising:

Two or more electrodes positioned on opposite sides of the plasma formation site for igniting the plasma prior to operation, the electrodes being positioned away from the longitudinal axis.

23. The apparatus of clause 22, wherein the electrodes are positioned on an axis normal to the longitudinal axis.

24. The apparatus of clause 23, wherein the electrodes, the drive system and the collection optics are arranged on three mutually orthogonal axes.

25. The apparatus of clause 16, further comprising a reflector positioned and shaped to reflect photons emerging longitudinally from opposite ends of the plasma back into the plasma.

26. The apparatus of clause 16, further comprising one or more reflectors positioned and shaped to emit light in one or more directions transverse to the longitudinal direction of the plasma photons are reflected back into the plasma.

27. A method comprising the steps of:

illuminating the structure with output radiation from a photon source device comprising;

a container configured to contain a gaseous environment;

a drive system configured to generate drive radiation and to form the drive radiation into at least one beam focused on a plasma formation region within the vessel; and

a collection optics system configured for collecting photons emitted by the plasma held at the plasma site by said radiation beam and for forming the collected photons into at least one output radiation beam,

wherein the drive optics are configured to maintain the plasma in an elongated shape having a length along a longitudinal axis substantially greater than in at least one direction transverse to the longitudinal axis the diameter of the plasma, and

wherein the collection optics are configured to collect photons exiting the plasma along a longitudinal axis from one end of the plasma;

detecting radiation diffracted by the structure; and

One or more properties of the structure are determined from the properties of the diffracted radiation.

28. An inspection device comprising:

a support for the substrate having the structure thereon;

an optical system for illuminating said structure under predetermined illumination conditions and for detecting a predetermined portion of radiation diffracted by the constituent target structure under said illumination conditions;

a processor arranged to process information characterizing the detected radiation to obtain a measure of a property of the structure;

Wherein said optical system comprises photon source equipment, said photon source equipment comprises:

a container configured to contain a gaseous environment;

a drive system configured to generate drive radiation and to transform the drive radiation into

become at least one beam focused on a plasma formation region within the vessel; and

a collection optics system configured to collect the plasma maintained by the radiation beam by passing

photons emitted by the plasma at the site and used to form the collected photons into

one less output radiation beam,

wherein the drive optics are configured to maintain the plasma in an elongated shape having a length along a longitudinal axis substantially greater than in at least one direction transverse to the longitudinal axis the diameter of the plasma, and

Wherein the collection optics are configured to collect photons emerging from the plasma along a longitudinal axis from one end of the plasma.

29. A photolithography system comprising:

Lithographic equipment, said lithographic equipment comprising:

an illumination optics system arranged to illuminate the pattern;

projection optics arranged to project an image of said pattern onto a substrate; and

Check equipment, including:

a support for the substrate having the structure thereon;

an optical system for illuminating said structure under predetermined illumination conditions and for detecting a predetermined portion of radiation diffracted by the constituent target structure under said illumination conditions;

a processor arranged to process information characterizing the detected radiation to obtain a measure of a property of the structure;

Wherein said optical system comprises photon source equipment, said photon source equipment comprises:

a container configured to contain a gaseous environment;

a drive system configured to generate drive radiation and to form the drive radiation into at least one beam focused on a plasma formation region within the vessel; and

a collection optics system configured for collecting photons emitted by the plasma held at the plasma site by said radiation beam and for forming the collected photons into at least one output radiation beam,

wherein the drive optics are configured to maintain the plasma in an elongated shape having a length along a longitudinal axis substantially greater than in at least one direction transverse to the longitudinal axis the diameter of the plasma, and

wherein the collection optics are configured to collect photons exiting the plasma along a longitudinal axis from one end of the plasma;

Wherein the lithographic apparatus is arranged to use measurements from the inspection apparatus when applying the pattern to other substrates.

30. A method of manufacturing a device, comprising:

Inspecting at least one composite target structure using an inspection method, wherein the at least one composite target structure is formed as part of a device pattern on at least one of the plurality of substrates or formed on at least one of the plurality of substrates Next to the device pattern, the inspection method includes:

illuminating the structure with output radiation from a photon source device comprising;

a container configured to contain a gaseous environment;

a drive system configured to generate drive radiation and to form the drive radiation into at least one beam focused on a plasma formation region within the vessel; and

a collection optics system configured for collecting photons emitted by the plasma held at the plasma site by said radiation beam and for forming the collected photons into at least one output radiation beam,

wherein the drive optics are configured to maintain the plasma in an elongated shape having a length along a longitudinal axis substantially greater than in at least one direction transverse to the longitudinal axis the diameter of the plasma, and

wherein the collection optics are configured to collect photons exiting the plasma along a longitudinal axis from one end of the plasma;

detecting radiation diffracted by the structure; and

determining one or more properties of the structure from properties of the diffracted radiation; and

A photolithography process for subsequent substrates is controlled according to the results of the inspection method.

Although specific references have been made above, using embodiments of the invention in the context of optical lithography, it should be noted that the invention may be used in other applications, such as imprint lithography, and as long as Allowed, not limited to optical lithography. In imprint lithography, the topology or topography in the patterning device defines the pattern produced on the substrate. The topography of the patterning device may be printed into a resist layer provided to the substrate whereupon the resist is cured by application of electromagnetic radiation, heat, pressure or a combination thereof. After the resist has cured, the patterning device is removed from the resist, leaving a pattern in the resist.

The terms "radiation" and "beam" as used herein include all types of electromagnetic radiation, including: ultraviolet radiation (UV) (e.g. having a wavelength at or about 365, 355, 248, 193, 157 or 126 nm) and extreme ultraviolet (EUV ) radiation (for example having a wavelength in the range of 5-20 nm), and particle beams, such as ion beams or electron beams. As mentioned above, the term "radiation" in the context of drive systems may also include microwave radiation.

Where the context allows, the term "lens" may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic, and electrostatic optical components.

The foregoing descriptions of specific embodiments will reveal the general properties of the invention sufficiently that others, by applying the knowledge of the art, can target various applications without undue experimentation and without departing from the general concept of the invention. Applications are readily modified and/or adapted to such specific embodiments. Therefore, such modifications and adaptations are intended to be within the range and meaning of equivalents of the disclosed embodiments, based on the teaching and suggestion presented herein. It should be understood that the terms or expressions herein are for the purpose of exemplification and description rather than limitation, so that the terms or expressions in this specification can be interpreted by those skilled in the art according to the teaching and inspiration.

The coverage and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the appended claims and their equivalents.

It should be appreciated that the Detailed Examples section, rather than the Summary and Abstract sections, is used to interpret the claims. The Summary and Abstract sections may set forth one or more, but not all, exemplary embodiments of the invention as contemplated by the inventors, and thus should not limit the invention and the appended claims in any way.

The invention has been described above with the aid of functional building blocks illustrating the implementation of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed.

The foregoing descriptions of specific embodiments will reveal the general properties of the invention sufficiently that others, by applying the knowledge of the art, can target various applications without undue experimentation and without departing from the general concept of the invention. Applications are readily modified and/or adapted to such specific embodiments. Therefore, such modifications and adaptations are intended to be within the range and meaning of equivalents of the disclosed embodiments, based on the teaching and suggestion presented herein. It should be understood that the terms or expressions herein are for the purpose of description rather than limitation, so that the terms or expressions in this specification can be interpreted by those skilled in the art according to the teaching and inspiration.

Claims (15)

1. A plasma-based photon source device comprising: (a) Containers used to contain gaseous environments; (b) a drive system for generating radiation, hereinafter referred to as drive radiation, and for forming said drive radiation into at least one beam focused on a plasma formation region within said vessel; as well as (c) a collection optics system for collecting photons emitted by the plasma held at the plasma site by said radiation beam and for forming the collected photons into at least one beam of output radiation, wherein the drive optics are configured to maintain the plasma in an elongated shape having a length along a longitudinal axis substantially greater than the length of the elongated shape transverse to the longitudinal axis diameter in at least one direction, and wherein the collection optics are configured to collect photons exiting the plasma from one end of the plasma along the longitudinal axis. 2. The photon source device of claim 1, wherein the drive system comprises at least one laser for generating the radiation beam. 3. A photon source device according to claim 1 or 2, wherein the drive radiation has a wavelength predominantly in a first range, such as an infrared wavelength, and the output radiation has a wavelength predominantly in a range different from the first range. Wavelengths in the second range, such as visible light and/or ultraviolet radiation. 4. The photon source device according to any one of claims 1-3, wherein the drive system is arranged for, along the longitudinal axis, a region of the plasma where the collected photons are emitted. The drive radiation is delivered at the end opposite to the one end. 5. The photon source device according to any one of claims 1-4, wherein the drive system is arranged to deliver the drive radiation to the plasma formation site in a direction transverse to the longitudinal direction. 6. A photon source device as claimed in claim 5, wherein the drive system is arranged for focusing the drive radiation into a substantially linear focus corresponding to the elongate shape of the plasma. 7. A photon source device according to any one of the preceding claims, further comprising two or more electrodes positioned on opposite sides of the plasma formation site for igniting the plasma prior to operation body, the electrodes are positioned away from the longitudinal axis. 8. The photon source device of claim 7, wherein the electrodes are positioned on an axis orthogonal to the longitudinal axis. 9. The photon source device according to claim 8, wherein the electrodes, the drive system, and the collection optical system are arranged on three axes that are orthogonal to each other. 10. A photon source apparatus according to any one of the preceding claims, further comprising a reflector positioned and shaped to reflect photons exiting longitudinally from opposite ends of the plasma back into the plasma. 11. A photon source device according to any one of the preceding claims, further comprising one or more reflectors positioned and shaped to move along a direction transverse to the longitudinal direction of the plasma. Photons exiting in one or more directions are reflected back into the plasma. 12. A method of measuring properties of a structure that has been formed on a substrate by a photolithographic process, said method comprising the steps of: (a) illuminating the structure with output radiation from a photon source according to any one of claims 1-11; (b) detecting radiation diffracted by the structure; and (c) determining one or more properties of the structure from properties of the diffracted radiation. 13. An inspection apparatus for measuring properties of structures on a substrate, the inspection apparatus comprising: (a) a support for a substrate having said structure thereon; (b) an optical system for illuminating said structure under predetermined illumination conditions and for detecting, under said illumination conditions, a predetermined portion of radiation diffracted by the constituent target structure; (c) a processor arranged to process information characterizing the detected radiation to obtain measurements of properties of the structure; (d) wherein the optical system comprises the photon source device according to any one of claims 1-11. 14. A photolithography system comprising: (a) lithographic equipment, said lithographic equipment comprising: (b) an illumination optics system arranged to illuminate the pattern; (c) a projection optics system arranged to project an image of said pattern onto a substrate; and (d) An inspection device as claimed in claim 13, Wherein the lithographic apparatus is arranged to use measurements from the inspection apparatus when applying the pattern to other substrates. 15. A method of fabricating a device in which a photolithographic process is used to apply a device pattern to a series of substrates, said method comprising the steps of: Inspecting at least one composite target structure using the inspection method according to claim 12, wherein the at least one composite target structure is formed as part of a device pattern on at least one of the substrates or is formed on the substrate Next to the device pattern on at least one of the substrates; and A photolithography process for subsequent substrates is controlled according to the results of the inspection method.